The present invention relates to bridge-output amplifiers and, more particularly, to degenerative feedback for such amplifiers.
A bridge-output amplifier employs four transistor means having their principal conduction paths connected in a bridge with the power supply being connected to one opposite pair of nodes and the load terminals being connected to respective ones of a second opposite pair of nodes. The transistor means in two opposing legs of the bridge have their conduction increased while the transistor means in the other two opposing legs either have their conduction decreased or are non-conductive. That is, the bridge output amplifier comprises first and second pairs of transistors, each pair of transistors having their principal conduction paths serially connected across the power supply and being operated as a push-pull amplifier to supply a separate one of the load terminals. Each of these component push-pull amplifiers responds to a common input signal in the opposite sense as the other to supply an output signal potential to its respective load terminal.
A principal advantage of the bridge-output amplifier is known to be that it permits the combined output signal potential swing between the load terminals to be twice the potential available from the power supply, so four times as much output power is available to the load as can be obtained with a pair of transistor means operated in push-pull. It is known that it is advantageous to differentially combine the signal potentials appearing at the load terminals of the bridge output amplifier to develop a feedback signal for degenerating the input signal controlling the relative conduction of the transistor means and thereby improve the linearity of the response of the difference of these signal potentials to the input signal.
Bridge-output amplifiers are known to be advantageous insofar as permitting the direct coupling of loads such as loudspeakers between their load terminals. Conventionally, the component push-pull amplifiers used in a bridge-output amplifier are adjusted to exhibit similar quiescent output signal potentials resulting in negligible quiescent current flow in the load. However, in certain bridge-output amplifiers such as those constructed in monolithic integrated circuit form it is infeasible to adjust the operating points of the component push-pull amplifiers. This presents the problem of reducing direct current flow through the load.